The long-term objective of this project is to describe and understand the development of auditory behavior in human infants. In the proposed studies, a rigorous test method is combined with psychophysical measures to describe the factors that influence infants' behavioral responses to sound. The proposed work has two specific aims. The first specific aim is to describe in detail the development of basic auditory processes during the first 6 months of life. Although the broad outlines of auditory development have been determined, the proposed studies will provide the more detailed information that is needed to fully understand the process of development. Understanding auditory development during this age period is crucial, as it is a critical period for early language development. Experiments are proposed to assess infants' ability to perceive the intensity and frequency of sound as well as changes in sound over time. The second specific aim is to determine how higher-level processes contribute to the maturation of hearing in infants older than 6 months of age. Experiments are proposed to determine how infants listen to the frequencies in a complex sound and to sounds that change over time, as well as how infants' listening is related to their auditory sensitivity. Of particular interest is how infants listen to short-duration sounds. Current models of infants' auditory processing cannot account for their apparent difficulty in detecting such sounds. Moreover, while deficits in the perception of short-duration sounds have been associated with language impairment in children, the normal developmental course of the perception of such sounds has not been well described. Experiments are proposed to describe infants' abilities to process short-duration sounds when they occur just before, just after or simultaneously with competing sounds. The effects of overlap between the frequencies comprising the short-duration sound and the competing sound will also be examined. Finally, an experiment is proposed to address how infants identify sound sources, because this process is another as yet unstudied auditory capacity that could contribute to infants' auditory sensitivity. The results of these studies will inform the process of identifying and remediation hearing and auditory perceptual deficits in infants and children.